Microwave diplexer



Dec. 8, 1959 N. F. ARTUso MICROWAVE DIPLEXER 3 Sheets-Sheet 2 Filed July9j 1954 INVENTR l /V/c//ULASF HTI/.s0

ATTORNEY ,United States Patent O IVIICROWAVE DIPLEXER Nicholas F.Artuso, Brooklyn, N.Y., assignor to Sperry Rand Corporation, acorporation of Delaware Application July 9, 1954, Serial No. 442,257

6 Claims'. (C1. 333-73) This invention relates to diplexer systems andmore particularly is concerned with apparatus for coup-ling twoelectromagnetic microwave signals of different frequencies to a singleelectromagnetic wave transmission line or for separating twoelectromagnetic microwave signals of different frequencies intodifferent electromagnetic wave transmission lines.

A diplexer system is disclosed in U.S. patent application Serial No.360,327 of K. Tomiyasu. Such microwave diplexer comprises in effect apair of four-terminal hybrid junctions having a conjugate pair of armsof each interconnected by two sections of wave guide transmission lineof different lengths. One of the line sections is of a xed length andthe other line section is of a substantially different and variablelength. Diplexing is achieved when the two frequencies and the lengthsof the two transmission lines satisfy two conditions of operation.First, the length of one of the transmission line sections must be anodd number of half guide wavelengths longer at one frequency than at theother frequency. Second, the length of the other transmission linesection must be an even number of half guide wavelengths longer at onefrequency than at the other frequency. Thus, the allowable change infrequency about a particular diplexing frequency (the bandwidth) is afunction of the path length between the two hybrid junctions. Thegreater the dierence in path lengths between the two hybrid couplers,the greater is the phase sensitivity between lthe two transmission linesas a function of frequency and the smaller is the operable bandwidth.

Another characteristic of the Tomiyasu diplexer is the method ofadjusting the system to achieve proper `diplexing action for a specifiedpair of frequencies. This adjustment is achieved by varying the lengthof the variable length transmission line until the specified twoconditions of operation occur.

It is therefore a principal object of this invention to provide adiplexer apparatus which improves upon the features of the Tomiyasudiplexer described above.

Another object of this invention is to provide a microwave diplexerwhose diplexing action is independent of the length differential of themicrowave transmission lines utilized. v

Another object of this invention is to provide a`diplexer system whichis capable of diplexing different pairs of operating frequencies withlittle adjustment elort. Another object of this invention is to providea diplexer system which is simple and inexpensive in construction.

Another object of this invention is the provision -for a microwavediplexer capable of operating at the high peak powers encountered in aradar transmission system.

Another object of the invention is to provide a diplexer apparatuswhichis operable over a relatively wide :band of frequencies.

4'Another object of this invention is to provide a diplexer whosebandwidth is determined 'solely-1 by Y the ice , 2 bandwidth of a cavityresonator having a relatively low loaded Q.

Another object of this invention is to provide a micro- Wave diplexerwhose bandwidth is determined solely by the bandwidth of a microwavefilter device.

Another object of this invention is to provide a microwave diplexerwhich may be tuned by the adjustment of a cavity resonator tuner.

Another object of this invention is to provide a nonreflectivefrequency-sensitive phase shifting means for use in a diplexer system.

In accordance with the present invention there is provided a system fordiplexing -two frequencies which comprises a pair of four-terminalhybrid junctions, one conjugate pair of each junction being connectedtogether by means of a pair of electromagnetic wave transmission lineswhose difference in electrical length remains constant. Inserted in oneof these transmission lines is means for causing the phase of one of thesignals to be shifted by an amount 180 different from that of the otherfrequency. This phase shifting means is adjustable in order thatdiiferent pairs of frequencies may be diplexed by the system.

Other objects and advantages of the present invention will becomeapparent from the specification taken in connection with theaccompanying drawings, wherein:

Figs. 1 and 2 are schematic views useful in explaining the operation ofthe diplexer of the present invention;

Fig. 3 is a schematic view of the non-reflective frequency-sensitivephase shift element used for diplexing action;

Fig. 4 is a graph showing the variation in reflection coelicient phaseangle as a function of frequency, for a cavity resonator;

Fig. 5 is a schematic view useful in explaining the preferred form ofthe invention;

Fig. 6 is a graph showing the combined phase shifts of the cavityresonator and phase shifting region of the structure of Fig. 5; and

Fig. 7 is a schematic view of a modified form of the invention in whichthe shunt cavity resonator elements are replaced by a tandem filter-typeelement.

Referring to the schematic drawing of Fig. 1, the numerals 21 and 22indicate a pair of electromagnetic wave transmission lines such ascoaxial transmission lines or wave guide transmission lines. The twotransmission line are coupled together by a pair of spaced hybridjunctions. These hybrid junctions may be either the magic tee type orthe hybrid directional coupler type.

A hybrid junction is an essentially lossless device which at highfrequencies may take the form of a metal enclosure at the junction offour transmission lines. The hybrid junction guides waves from one toanother of the four lines in a particular way. A fundamental prop- `ertyof the junction is that when its four arms are comoutputs will depend onthe type of hybrid junction used.

Several types of hybrid junctions are well known in the art; inparticular, the magic tee andthe hybrid directional coupler. In themagic tee, the relative phase angles of the two equal output powers areseparated by either 0 or depending on the arm into which the power isinjected. In the instance of the hybrid directional coupler, regardlessof thearm into which the power is in- Patented Dec. 8, 1959 vboth wavesin passing the hybrid junction.

Jected, the relative phase between the two equal output powers willalways be 90.

For illustrative purposes, hybrid directional couplers 23 and 24 areindicated in Fig. 1. In the hybrid ldirec- 'tional coupler energyentering any one of the four arms will leave equally the two oppositeoutput arms. 'While the energy leaving the two output arms `is equal,the phase angle of that portion of the energy which passes through thecoupling element, `known as the coupled wave, will lag by 90 the phaseangle of that energy which does not pass through the couplingelement,'known as the direct wave.

ln operation, an electromagnetic signal of frequency F1 vwith arelativeelectric vector amplitude of unity and a phase angle of is shownentering an arm 25 of `hybrid directional coupler 23. The energy dividesequally at 'the coupler 23, half of it (termed the direct wave) passingdirectly into ktransmissionline 21, and the other half (termed thecoupled wave) passing through the 'coupling element of the hybriddirectional coupler into transmission line 22. The amplitude of thedirect wave will have a value of at a relative phase angle of 0, whilethe amplitude of ,the coupled wave will have an amplitude of at an angleof'90 lagging. It is to be noted that an equal additional phase delay of45 is introduced into This will not be considered here since it plays nopartin the operation of the device. A frequency sensitive phase shiftelement 29 inserted in transmission line 22 is designed to produce nophase delay for signals of vfrequency F1.

-Since the transmission lines '21 and 22 are equal in length between thecouplers 23 and 24 they will produce the same electrical phase delay ofboth the direct and coupled waves, and consequently the direct andcoupled waves will reach the input end of the hybrid directional coupler24 with their relative phase relationship unchanged.

The energy entering the coupler 24 from transmission line 21 `Jvilldivide equally, half of it passing into the arm 27 and half passing intothe arm 28. The portion passing into arm 27 will have an amplitude ofone-half and a relative phase angle of 0. The portion entering arm 23will have an amplitude one-half and a relative phase angle of 90. Thesignal entering coupler 24 from transmission line 22 will also divideequally, half entering the arm 27 and half entering the arm 28. Theportion entering the arm 28 will be unchanged in phase, remaining at arelative angle of 90 and having an amplitude of one-half. The portionentering arm 27 will be further retarded in phase by 90, thereby havinga phase angle of -180 and an amplitude of one-half. The two signalsentering arm 27 are equal in amplitude but opposite in phase and therebycancel, resulting in no energy of frequency F1 leaving arm 27. The twosignals entering arm 28 are alike in phase and add, resulting in asignal of frequency F1 leaving arm 28. Thus, all of the energy whichentered the arm 25 at frequency F1 will leave the diplexer at arm 28 andnone will leave arm 27.

Consider now Fig. 2 in which the energy entering `arm 2S is at adifferent frequency F2. For energyrat this frequency, the phase `shiftelement 29 will produce a phase lag of 180 as energy passes through orVby the element. The frequency F2 entering :arm 25 has unity amplitudeand a relative yphase'angle'of 0. This signal divides equally at thehybriddirectional coupler 23, half of it passing into transmission-line21 and-half into transmission line 22. The portion passing into line 21will have an amplitude of 'at a. relative phase angle of 0. into line 22will have an, amplitude at a relative phase angle of Since the two lines21 and 22 are equal in length between the directional couplers 23 and24, the only difference in the relative phase of the two Wavespropagating along these arms will be caused by the phase delay insertedby phase shift element 29. Since the wave propagating in line 22 enteredthat line lagging the wave propagating in line 21 by 90, it will furtherlag that wave by or a total of 270, when it reaches the directionalcoupler 24. For convenience this wave in line 22 is shown arriving atcoupler 24 at a phase angle +90 rather than a phase angle of 270, Theenergy from line 21 entering coupler 24 will divide equally `and passinto arms 27 and 28. That portion entering arm 27 will have an amplitudeof one-half and a relative phase angle of 0. That portion entering arm28 will have an amplitude of one-half and a relative phase angle of 90.The energy entering the coupler 24 from line 22 will divide equally,half passing out arm 27 and half out arm 28. The portion leaving arm 28has an amplitude of one-half and a relative phase angle of 90. Theportion leaving arm 27 is further delayed in phase by 90 and leaves at arelative phase angle of 0 and an amplitude of one-half. The two signalsleaving arm 27 are equal in amplitude and alike in phase so that asignal of amplitude unity at the frequency F2 leaves arm ,27. The twosignals leaving arm 28 are equal in amplitude but opposite in phase andthereby cancel, so that -no energy leaves arm 28. Thus all of the signalwhich entered arm 25 at frequency F2 will leave the diplexer at arm 27and none will leave arm 28.

If the operation of Figs. 1 and 2 are combined, such that vsignals offrequency F1 and F2 enter the diplexer at arm 25, it is seen that all ofthe energy of frequency F1 will leave arm '28 and all of the signal offrequency F2 will leave arm 27, the two signals thereby beingvseparated.

A preferred form of the frequency sensitive phase shift element is shownschematically in Fig. 3. Two electromagnetic wave transmission lines 41and 42 are coupled together by means of a hybrid junction such as thedirectional coupler 43. Microwave energy enters the device through arm`44 and leaves through arm 45. The energy leaving arm 45 differs inphase from that entering by an amount which depends on the frequency oftransmission. Arms 44 and 45 constitute a conjugate pair of arms.Connected to the second conjugate pair of arms 46 and 47 are a pair ofresonant cavities 48 and 49. The resonant cavities 48 and 49 areseparated from the arms 46 and 47 by the respective apertured walls 50and 51 which contain the respective apertures 52 and 53.

If a wave external to the cavity is incident on apertured wall 50 or`51, it will be reflected with unchanged magnitude but with a phaseangle which is a function of frequency. A curve of the reflectioncoeicient angle for such a wave as a function of the fractionaldeviation from the resonant frequency of the cavity is shown in Fig. 4.The curves plotted are for different values of cavity GQ which representthe product of the normalized shunt conductance G times the Q yof thecavity.

To analyze this frequency sensitive phase shift element assume a wave ofamplitude unity at relative phase angle 0 entering arm 44. For thisanalysis, the 45 `phase lag of the direct wave Vdue to the coupler-isincluded in the The portion passing assigned phase angles. This wavewill divide equally at hybrid coupler 43 half entering arm 46 and halfentering arm 47. The portion entering arm 46 has an amplitude 1 at aphase angle 45. The portion entering arm 47 has an amplitude at a phaseangle 135 due to the additional 90 phase delay caused by passing throughthe coupling element. The Waves in arms 46 and 47 directed toward therespective resonant cavities 48 and 49 are reflected therefrom andreturn along their prior paths unchanged in magnitude, but furtherretarded in phase by the phase angle 6 of the reflection coeicient, asindicated in Fig. 4. The reflected wave in arm 46 divides at hybridcoupler 43 half entering arm 44 and half entering arm 45. The portionentering arm 44 has an amplitude of one-half at a phase angle 90-Hi. Theportion entering arm 45 has an amplitude one-half at a phase angle180-l-0. The reflected wave in arm 47 divides equally at hybrid coupler43 half entering arm 44 and half entering arm 45. The portion enteringarm 44 has an amplitude one-half at a phase angle 270-+0. The portionentering arm 45 has an amplitude one-half at a phase angle -180|0. Thetwo wave portions which tend to return in arm 44 are equal in amplitudebut opposite in phase and consequently cancel, no energy propagatingbackward along arm 44. The two wave portions entering arm 45 are equalin amplitude and alike in phase, thereby adding and causing a wave ofamplitude unity at a relative phase angle -l80|0 to leave arm 45. Theunit thus described transmits all the energy from the input arm to theoutput arm with no energy being reflected, the energy leaving the outputarm bearing a phase relative to that entering the input arm which isdependent on the frequency of the energy. This, therefore, is anOn-reective frequency sensitive phase shifter.

Fig. 5 illustrates a schematic diagram of an embodiment of thisinvention. The numerals 62, 64, 66 and 68 indicate sections of hollowrectangular wave guide. The numerals 70, 72, 74 and 76 indicate hybriddirectional couplers. A suitable hybrid directional coupler is describedin the Proceedings of the I.R.E., February 1952, page 180. The waveguide sections 62 and 64 are coupled together near one of theirextremities by the hybrid directional coupler 70. In a like manner thewave guide sections 64 and 68, 68 and 66, and 62 and 66 are coupledtogether by the respective hybrid directional couplers 76, 72 and 74.The Wave guide sections 62 and 64 are connected to the respective inputarms 78 and 80. The wave guide sections 66 and 68 are connected to therespective output arms 82 and 84. Connected to one end of the hybriddirectional coupler 76 are resonant cavities 86 and 88. Energy iscoupled to resonant cavities 86 and 88 by means of the respectivecoupling apertures 90 and 92. The wave guide sections 64 and 68, thehybrid coupler 76, and the cavities 86 and 88 comprise a frequencysensitive phase shift element as heretofore described. Corinected to oneend of the hybrid directional coupler 74 are wave guide short circuits94 and 96.

4"Anon-reflecting energy absorbing termination is indi-l cated generallyat 97 in the input arm 80. Energy absorbing material such as polyiron ora mixture of graphite and cement, indicated at 98, is inserted in theinput arm 80 to provide such non-'reflective termination. A source ofenergy 99 of two different frequencies F1 and F2 is connected to theinput arm 78. A utilization circuit 100 for a signal of frequnecy F2 isconnected to the output arm 82 and a utilization circuit 102 for asignal of frequency F1 is connected to the output arm 84. Suchutilization circuit may, for example, be a receiver.

Cat

The wave guide short circuits 94 and 96 must be so positioned that thetotal length of the wave guide sections 64 and 68 between the hybriddirectional couplers 70 and 72 must be substantially equal to the totallength of the wave guide sections 62 and 66 between the same hybriddirectional couplers. With the wave guide shorts 94 and 96 so adjustedthe phase difference of the two waves arriving at coupler 72 from waveguides 66 and 68 will be independent ofthe absolute lengths of the waveguide sections between the couplers 70 and 72 for a substantial range offrequencies. This phase difference is not dependent on any criticalguide wavelengths which vary with frequency, but is due solely to thedifference introduced at coupler 70 and any artificially introducedphase shifts which cause the phase delay along one path to be differentfrom that along the other path. Recalling from the analysis of Figs. 1and 2, that diplexing action will be achieved if the phase shift element29 produces a phase delay which is different at one of the frequenciesthan at the other, it has been found that for greatest bandwidthoperation if resonant cavities, which have the characteristics shown inthe curve of Fig. 4, are utilized for the phase shift element, thediplexing frequency is preferably selected at those points on the curvewhich indicate reflection coeicient angles of +90 and -90, although anytwo points which differ by 180 degrees may be selected. v

Thus for the lower frequency F1, the wave entering wave guide 64 lagsthat of wave guide 62 by 90 and will arrive at the dotted section XX ofwave guide 68 lagging the wave in wave guide 66 arriving at the samesection, by an additional 270. The phase difference between these twowaves of wave guides 66 and 68, for the lower frequency F1, on arrivingat dotted section XX is 0. By a similar analysis the higher frequency F2arriving at the dotted section XX in wave guide section 68 lags that inwave guide 66 by 180. -If the signals at these two frequencies werepermitted to reach coupler 72 with their relative phase anglesunchanged, no diplexing action would occur, but instead equal energywould pass out both output arms 82 and 84 for both frequencies. Tocorrect for this difficulty, means m-ust be included in one of the waveguide sections 62, 64, 66 or 68 to cause the electrical length of saidsection to differ by 90 from the electrical length of the othersections. Such a scheme is shown at region 104. In this region, the waveguide section 68 is changed in form so as to cause its electrical lengthto differ from that of wave guide 66 by 90 for the entire band offrequencies at which the diplexer operates. The particular meansutilized is to decrease the width of the wave guide 68. Other means forobtaining this 90 phase shift may be used, such as dielectric loading ofa wave guide section or ridge loading of a wave guide section.

This added phase shift means in wave guide section 68 introduces anadded 90 phase difference between the respective waves propagating inwave guides 68 and 66. The energy of the lower frequency F1 beingdiplexed reaching directional coupler 72 from wave guide section 68 lagsin phase by 90 that'reaching the coupler from arm 66 for the embodimentshown. This lower frequency F1 will then leave the diplexer at outputarm 84. The signal at the upper frequency F2 will now arrive at coupler721r from the wave guide section 68 lagging by 270 the energy of waveguide section 66. Therefore, the energy at this upper frequency F2 willall leave the diplexer at output arm 82. Fig. 6 shows a curve of thecombined phase delays of region 104 and that of resonant cavities 86 and88. It is noted that at the diplexing frequencies this phase differenceis the required 0 and 180.

To determine an appropriate bandwidth for a diplexer, a standard must beestablished. A useful standard is the frequency range over which thephase difference of the signals arriving at the second hybrid coupler 72does not deviate by more than 30 from the required phase difference fordiplexing. Within this 60 frequency band, centered about the diplexingfrequency, the insertion loss for the transmitted signal will always -bebelow 0.5 db, allowing for 0.2 db losses -in the Wave guide walls andspurious reflections.

This standard is indicated by the boundaries yof the shaded areas ofFig. 6. An analysis ofthe systemfshows that-the bandwidth achieved bythis method is two-.thirds the frequency difference between the resonant'frequency fo of the cavity and the extreme ends of the diplexing range.

In designing the resonant cavities for this system, the resonantfrequency fo is selected midway 'between the desired diplexedfrequencies. To determine the required Q of the cavity it is thennecessary merely to determine that cavity whose Q will yield .reflectioncoefficient angles of +90 and 90 at the respective frequencies to -bediplexed. Solution of the equation for the product GQ, where equals 90,will supply 'the necessary information. The normalized shunt,conductance G is determined from the allowable losses.

To enable this device to diplex different frequencies, the resonantcavities may be made tunable. 'Ehe use 0f cavity tuning means such as:adjustable shorts or plungers will `allow changing the cavity resonantfrequencies and cavity Q, and thereby permit rapid changing of thediplexed frequencies.

By considering the well-known principles of reciprocity, it may be seenthat if a source of energy of frequency F1 is connected to arm 84 andasource Vof .energy of frequency F2 is connected to arm 82, the twosignals will be combined by this device so as to leave 'the diplexer atarm 78. Thus, this invention may be utilized for combining into oneelectromagnetic wave transmission line signals from two independenttransmission lines.

An alternative embodiment of this invention is disclosed in Fig. 7. Inthis form of the invention the frequency sensitive phase shift element229 is a kwave guide filter section. The frequency sensitive phase shiftelement 229 is coupled to wave guide section 222, Awhich in turn iscoupled to wave guide section 221 by the lseparated hybrid directionalcouplers 223 and 224. Known inthe art are many forms of wave guidefilter sections which will produce a varying phase shift las a functionof frequency in the pass band so as to satisfy the requisites of thisdevice In addition, such 4filters are known which can have a relativeimpedance equal to the characteristic impedance of the wave `guide overthe band of y-frequencies. An example of such a filter is an m-derivedband pass filter.

Since many changes could be made -in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing yfrom the scope thereof, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings, shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

1. A non-reliective frequency sensitive phase shifter comprising ahybrid directional coupler, two electromagnetic wave transmission linesections having first and second ends coupled together intermediatetheir ends by saidcoupler, and two similar cavity resonators coupled tothe first ends lof said line sections.

2. A diplexer transmission system comprising four hybrid junctions, eachjunction having two conjugate pairs of arms, four hollow rectangularwaveguide sections, each section coupling the first arm of the firstconjugate pair of arms of one of said junctions to the second arm of thefirst conjugate pair of arms of another of said junctions, a pair ofresonant cavities coupled to the second conjugate pair of arms of one ofthe hybrid junctions, and a phase shifting means in one of said hollowrectangular waveguide sections.

3.'A diplexer transmission system comprising .three hollow rectangularwaveguide sections, a first hybrid junction coupling the first andsecond of said waveguide sections, a second hybrid junction coupling thefirst and third of said waveguide sections, a third hybrid junction,said second and third waveguide sections being coupled to one conjugatepair of arms of said third hybrid junction, two cavity resonators, onecoupled to each of the arms of the other conjugate pair of arms of saidthird hybrid junction, and a 90 phase shifting means in one of saidhollow rectangular waveguide sections.

4. A diplexer transmisison system comprising four hybrid junctions, eachjunction having two conjugate pairs of arms, four hollow rectangularwaveguide sections, each section coupling the first arm of the firstconjugate pair of arms of one of said junctions to the second arm of thefirst conjugate pair of arms of another of said junctions, a pair ofresonant cavities coupled to the second conjugate pair of arms of one ofthe junctions, shorting means connected to the second conjugate pair ofarm-s of another of said junctions, and a 90 yphase shifting means inone of said hollow rectangular waveguide sections.

5. A diplexer transmisison system comprising -two hybrid junctions,first and second hollow rectangular waveguide sections coupled togetherat each of `two separated positions by means of said hybrid junctions, afrequency sensitive phase shifting means positioned in one of the waveguide sections intermediate said junctions, said means including thirdand fourth hollow rectangular waveguide sections connected by a thirdhybrid junction, each of said waveguide sections having first and secondends, cavity resonator means connected to the first end of each of saidthird and fourth waveguide sections, `the second ends of said third andfourth waveguide sections being connected into the first waveguidesection, and Va phase shifting means in one of said hollow rectangularwaveguide sections.

6. A non-reflective frequency sensitive phase shifter comprising ahybrid directional coupler having two conjugate pairs of arms and a pairof similar cavity resonators connected to one conjugate pair of arms.

References Cited in the file of this patent UNITED STATES PATENTS2,564,030 Purcell Aug. 14, 1951 2,593,120 Dicke Apr. 15, 1952 2,595,680Lewis May 6, 1952 2,632,809 Riblet Mar. 24, 1953 2,679,631 Korman May25, 1954 2,702,371 Sunstein Feb. 15, 1955

